1. Field of the Invention
This invention generally relates to a plasma source, and in particular to a bipolar plasma source made up of two hollow cathodes to which is applied a bipolar AC signal for driving the hollow cathodes to mutually opposed positive and negative voltages in order to generate a uniform plasma.
The invention also relates to a method of generating plasmas by applying a bipolar AC signal to two hollow cathode structures in order to generate a uniform plasma.
Applications of the bipolar plasma source of the invention include (i) an effusion cell in which heat used to evaporate the evaporant material is directly or indirectly supplied by plasma generated within two opposed hollow cathodes powered by a bipolar power source, the hollow cathodes being situated in a lid of the effusion cell adjacent an exit through which vapor exits the effusion cell, and (ii) a plasma sheet source in which the plasma is generated by two opposed hollow cathode slots powered by a bipolar power source, the plasma being confined by magnetic field lines running parallel to the electric field lines, the plasma sheet source being suitable for use in energizing a gas situated between a vacuum deposition source and a substrate to be coated.
2. Description of Related Art
The present invention seeks to provide a more uniform and stable plasma, in order to enable use of the plasma as a heat or energy source in processes requiring even application of energy or heat over an extended area. Processes to which the plasma source and method of the invention may be applied include coating processes such as vacuum deposition, the plasmas being used as a heat source for an evaporant in an effusion cell and/or to apply energy to a gas situated between the vacuum deposition source and a substrate to be coated.
A. Background Concerning Hollow Cathode Plasma Generation
The present invention achieves the stable and uniform plasma necessary for applications requiring, for example, efficient heating of an evaporant in an effusion cell or uniform energization of a reactant, by using a modification of the conventional plasma generation method known as hollow cathode plasma generation. While uniform plasmas have been generated using closed drift ion sources having a closed path plasma shape, including sputtering magnetrons, such sources are bulky and not readily adaptable for use with conventional vacuum or sputter deposition apparatus. Conventional hollow cathode plasma devices, in contrast, are simple, compact, and efficient, but it has heretofore been impossible to generate a stable and uniform plasma over an extended linear dimension using a hollow cathode plasma generator, and thus use of hollow cathode plasma generators as heat or energy sources for coating processes has been limited.
A hollow cathode is simply a cavity in the form of an opening or slot in a conductive material. When a voltage is applied to the conductive material, the applied voltage will cause electrons in a gas present in the opening or slot to acquire energy from the applied voltage, eventually resulting in formation of a plasma.
Key parameters in the formation of plasmas include the material and pressure of the gas present in the opening or slot that forms the hollow cathode, the shape and dimensions of the opening or slot, and the material of the walls that define the opening or slot. In addition, the internal diameter of a cylindrically shaped opening or the width of the slot required to sustain a discharge depends on the gas pressure, so that the ideal cavity size for the plasma is inversely proportional to the pressure so that the higher the operating pressure the smaller the cavity. Common cavity diameters or slot widths are on the order of 1 mm to 3 mm at gas pressure in the range of 10xe2x88x921 to 10xe2x88x923 Torr although devices that operate from atmospheric pressure down to the 10xe2x88x924 region have been proposed and are intended to be included within the scope of the present invention.
Depending on the above-described parameters, the plasmas formed in the opening or slot may be used in a wide variety of industrial applications, including applications based on the energy transfer involved in plasma formation for such surface treatment applications as cleaning, etching, and activation of compounds present in or on the surface, as well as deposition applications involving transfer of materials, which may include materials from the walls of the cathode, for transport to a surface. The constituents of the plasmas may be transported from the hollow cathode and accelerated through a nozzle by electric fields or other transport phenomena, or the plasmas may be allowed to lose energy shortly after formation through the recombination of electrons with ions.
While the hollow cathode plasma formation technique is well-known and has successfully been used for many years, it is difficult to use the hollow cathode plasma formation technique to form plasmas covering an extended area. In addition, the hollow cathode plasma formation technique has the disadvantage in some applications that the electrodes are subject to contamination by vapors.
The conventional method of generating plasmas covering an extended linear dimensions is to form an array multiple hollow cathodes in a common conductive structure to which cathode the voltage may be applied. Ideally, if the openings or slots are of uniform dimensions, then each cathode should simultaneously generate an identical amount and/or density of plasma, resulting in a relatively uniform formation of plasma over the extended area. However, even under ideal conditions, the hollow cathode array method of generating plasmas suffer from a number of problems:
First, the plasma formed by such an array cannot be completely uniform due to the discrete nature of the cathodes in the array.
Second, a problem arises in that unless the cathodes all light simultaneously, the ignition of some cathodes will cause a voltage drop in the area of other cathodes that may eventually prevent their ignition, making it very difficult in practice to get each hole to ignite uniformly.
Third, in any practical application, the walls of the cathodes will become contaminated, causing variations in the fields generated within the cathodes by applied voltages.
One way to counteract the inherently non-uniform nature of plasmas generated in discrete cathode arrays, and also the effects of differences in the electrical characteristics of the openings, including those resulting from contamination, is to use magnetic fields to cause the plasma to spread out more evenly following formation. The magnetic fields used for this purpose are in addition to any magnetic or electric fields used to manipulate or control the plasmas for a particular application following generation of the plasmas, however, and therefore add to the complexity of the device. In addition, it can be difficult to control over extended areas or beam lengths.
The present application avoids the problems of plasma instability and non-uniformity by utilizing a technique similar to that used to prevent accumulation of materials during sputter deposition, namely the use of two magnetrons to which an AC current is alternately applied, with the positive cycle of the AC current effectively discharging contaminants from the plasma source. In the case of sputter deposition, the technique involves the use of dual magnetrons or alternately energized cathodes, whereas the present invention involves the use of hollow cathodes.
It turns out that during hollow cathode plasma formation, the effect of applying an AC voltage to a pair of cathodes is not only to reduce the effect of contaminant formation in the cathodes (such as unintended arcing), but surprisingly also to increase the uniformity of the plasma over an extended linear area, permitting the formation of plasmas having an indefinite linear dimension. Even though each individual hollow cathode arranged in the manner to be described below may be identical to a conventional hollow cathode, the conventional hollow cathode, even when magnetic fields are added to enhance uniformity, cannot come close to generating plasmas as uniform over an extended distance as those obtained by the invention. Furthermore, in contrast to conventional hollow cathode arrangements, the present invention permits formation of both xe2x80x9cnormalxe2x80x9d and thermionic plasmas, as will also be explained below,
As indicated above, the stability and uniformity of plasmas is critical in many industrial applications, and thus numerous prior attempts have been made to improve the uniformity or linearity of plasmas. Background patents directed to improving the uniformity of plasmas created by hollow cathode techniques, or of generating plasmas over an extended area using arrays of hollow cathodes, include by way of example U.S. Pat. Nos. 5,908,602, 5,217,761, and 5,627,435, as well as in the publications entitled High Density Plasma Sources (Noyes Publications, pages 413-418)and xe2x80x9cMulti-jet Hollow Cathode Discharge for Remote Polymer Deposition,xe2x80x9d in Surface Coatings and Technology (vol. 93, 1997, pages 128-133), while devices or techniques directed to improving the uniformity of ion beams, which are similar to plasmas but lack the free electrons, are disclosed in U.S. Pat. Nos. 6,002,208, 5,763,989, 5,359,258, and 4,862,032, and sputter deposition arrangements using magnetrons are disclosed or discussed in U.S. Pat. Nos. 5,944,967 and 5,897,753. Additional background on hollow cathodes or plasma generation techniques in general are found in U.S. Pat. Nos. 6,064,156, 6,037,717, 5,969,470, 5,939,829, 5,938,854, 5,917,286, 5,457,298, 5,414,324, 5,437,778, 5,387,842, 5,241,243, 5,075,243, and 5,075,594.
None of the linear plasma or ion sources described in the above patents and literature results in the generation of a continuous plasma that is as uniform as that provided by the present invention, and yet most of the linear plasma or ion sources described there in are far more complex than that of the present invention, with the except of simple arrays of small plasma sources which, as discussed above, are non-continuous and have problems with achieving equal plasma intensities in all holes.
B. Background Concerning Plasma Sheet Generation
One application for the plasma generator of the invention is as an energy source for increasing the reactivity of a reactant gas such as oxygen or nitrogen used to form oxides or nitrides of silicon, titanium, or other metals emitted by a vacuum deposition source. Typically, in such coating processes, the reactive gas in introduced into the space between the metal source and the substrate. As the metal atoms impinge on the substrate, the reactive gas atoms are also present at the surface and a chemical reaction takes place to create a metal compound coating. The heat of the reaction is absorbed by the substrate atoms to allow the metal and reactant gas atoms to stay combined.
In order to enhance the reaction rate between the metal and the reactive gas and thereby ensure a fully oxidized or stoichiometric compound, the energy level or reactivity of the reactive gas may be raised by an order of magnitude or more if the gas is transformed into a plasma. The resulting disassociated forms of the reactive gas molecules, neutral gas atoms or molecule with electrons in higher orbits, and gas ions are all more reactive than the gas in its ground state, and the higher the ratio of these activated species to ground state gas atoms, the more reactive the plasma.
For large area sputtering or vacuum deposition, such as on polymer webs or glass, the uniformity of the deposited layer is usually critical. Required measurements of thickness, layer morphology, or layer oxidation state across the width of the substrate must often be held to less than xc2x12%. To meet these requirements in a plasma assisted process, not only must the deposition rate from the coating source be highly uniform, but so must the plasma density created by the plasma generation apparatus.
Achieving uniformity is made more difficult in this application not only because of the linear dimension of the sputtered or evaporated metal that must be combined with the reactant gas, but also the area. In the case of a plasma generator, the distance between the anode and cathode must be at least 10 cm wide. It is very difficult to achieve a uniform plasma over such an extended area using conventional apparatus. The bipolar plasma of the present invention, on the other hand, is ideally suited to providing the highly uniform plasma necessary to such a plasma-assisted coating process.
C. Background Concerning Effusion Source Heating
In addition to providing a continuous plasma source that can be applied to energize a reactive gas used in vacuum deposition, the invention can be used to provide an improved effusion source for the vapor flux used in the deposition, by utilizing a plasma rather than a resistive heat source to heat the evaporant in the plasma. In this application, the plasma serves to provide a more efficient heat source while reducing the size of the effusion source and increasing its reliability.
A conventional effusion source, also known as an effusion cell or a Knudsen cell, typically consists of a cylindrical container positioned in a vacuum chamber, a lid for the container having a hole smaller than the inside diameter of the cylinder, a material inside the container to be evaporated, and a resistive heat source to heat the evaporant material. When the evaporant material is heated to sufficient temperature, some of the evaporant changes state to the gas phase, which raises the pressure inside the cell to higher than the background vacuum pressure. The vapor in the cell at this time may be chemically the same as the evaporant, or disassociation may occur when the evaporant is a compound. In either case, due to the higher pressure in the cell, the vapor exits the lid hole and is directed at a substrate upon which the vapor condenses forming a coating. Generally, the whole cell must be heated as well as the evaporant. If a section of the hole is cooler than the vapor, condensation will occur at the hole which may consequently become plugged or partially plugged.
The shape of the effusion cell may be modified to suit the substrate which is being coated. For the purpose of coating polymer webs or other large substrates, the source may be an elongated rectangular box fitted with an elongated lid having a slot or a series of holes for the exiting vapor. The heat for vaporizing the evaporant material is supplied by resistively heated elements which may either be placed withing the crucible, formed by the lid itself as described in U.S. Pat. No. 5,902,634, positioned outside the crucible as described in U.S. Pat. No. 5,167,984, or in an attached secondary box as described in U.S. Pat. No. 4,401,052.
Each of the prior resistively heated effusion cell designs requires a high current, in the range of hundreds of amperes, to reach the high temperatures necessary to achieve a sufficient vaporization rate. As a result, parts of the heating apparatus other than the heating elements themselves must be water cooled. This results in the problems that it is difficult to achieve a uniform temperature, and that the water cooled parts to which the heating element is connected drain heat away from the heating elements. In addition, lids or other elements arranged to serve as resistors tend to be subject to breakage at high temperatures, necessitating immediate replacement of the resistive heating elements. The use of a bipolar plasma source significantly reduces each of these problems.
It is accordingly a first objective of the invention to provide a continuous, stable, and uniform plasma, and yet that is simple in construction.
It is a second objective of the invention to provide a continuous, uniform plasma over an extended linear dimension.
It is a third objective of the invention to provide a plasma source that provides improved uniformity without the need for magnetic fields to distribute the plasma more uniformly during formation.
It is a fourth objective of the invention to provide a plasma source for use as a heat source that, in comparison with a resistive heat source, does not require a bulky high current power supply and connectors.
It is a fifth objective of the invention to provide a plasma source having a relatively small profile, that can be retrofit in vacuum chambers where larger sources do not fit.
It is a sixth objective of the invention to provide a plasma source having improved resistance to arcing even when contaminated or dirty.
It is a seventh objective of the invention to provide a plasma source having a simple construction that can nevertheless be run in both normal mode and thermionic mode.
It is a eighth objective of the invention to provide a plasma source having a simple construction and yet that is more versatile than prior sources, and that can be used in many different configurations.
It is a ninth objective of the invention to provide a plasma source that provides uniform, stable, and quiet plasmas from a variety of gases that would otherwise contaminate the source, including SiO, TiO, and other oxides or nitrides such as are used in coating processes.
It is a tenth objective of the invention to provide various plasma generation applications which make use of a more uniform, continuous plasma generator, including:
A. a plasma sheet source suitable for uniformly energizing a reactive gas introduced between the vacuum deposition source and a substrate;
B. a plasma assisted reactive process utilizing the more uniform plasma sheet source; and
C. an effusion cell type evaporation source where the heat is derived from a plasma rather than a resistive heating effect.
These objectives are achieved, in accordance with the principles of a preferred embodiment of the invention, by providing a structure including first and second hollow cathode shapes to which is applied a bipolar alternating current output power supply (the term alternating current referring to any current that reverses polarity, including sine waves, square waves, or any other periodic or aperiodic alternating waveform). The bipolar power supply initially drives the first hollow cathode shape to a negative voltage, allowing plasma formation while the second hollow cathode shape is driven to a positive voltage in order to serve as an anode for the voltage application circuit, and then drives the first hollow cathode shape to a positive voltage, reversing the roles of cathode and anode.
The hollow cathode arrangement of the preferred embodiment of the invention may operate in either of two general modes, normal and thermionic. In normal mode, the hollow cathode temperature is kept low by water cooling or other cooling methods, and the cathode requires a few hundred to a few thousand volts to operate, with electron current remaining relatively low. In thermionic mode, the hollow cathode is allowed to rise in temperature from plasma heating effects or from a separate heating device, such that when the hollow cathode surface reaches very high temperatures, electron emission rises an order of magnitude higher than that of a cold cathode, resulting in a high cathode discharge current (e.g., 100 amps) at a relatively low voltage (e.g., tens of volts). The temperature required to reach thermionic mode and the electron current required is dependent on the work function of the material of the cathode.
The plasma source of the preferred embodiment of the invention may be used, without limitation, in any of a variety of applications in which stable and uniform plasmas are required. The present invention provides two specific examples of such applications.
The first example involves use of the hollow cathodes as a heater for an effusion cell used in vapor deposition. In this application, the hollow cathodes are placed on opposite edges of the slot through which vapor exits the cell, the plasma being used to directly or indirectly heat the vapor and/or the evaporant, either before or during exit of the vapor from the effusion cell.
The second example of an application for the plasma source of the invention is as a plasma sheet source which may be used to energize a reactant gas during a vacuum deposition coating process, or which may be formed from the reactant gas. The plasma sheet generator includes two approximately facing hollow cathodes powered by an AC power supply, as described above, which are situated between the vacuum deposition source and a substrate to be coated, and which transforms the reactant gas into a plasma to improve the reactivity of the gas so as improve the uniformity of the deposited compound.
According to an especially preferred implementation of the plasma sheet source, which may be used in application other than for the purpose of energizing a reactant in a coating process, a magnetic field parallel to the electric field between the hollow cathodes is used to restrict the electrons to a path between the hollow cathode shapes, thereby enabling establishment of a greater current between the shapes, and permitting the shapes to be positioned a greater distance apart. The magnetic fields may be planar, or the magnetic fields may be curved to permit use in applications where the hollow cathodes do not face each other.